In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is burned, and the hot combustion gases are passed through a turbine mounted on the same shaft. The flow of combustion gas turns the turbine, which turns the shaft and provides power to the compressor and to the fan. In a more complex version of the gas-turbine engine, the compressor and a high-pressure turbine are mounted on one shaft having a first set of turbines, and the fan and a low-pressure turbine are mounted on a separate shaft having a second set of turbines. The hot exhaust gases and the air propelled by the fan flow from the back of the engine, driving it and the aircraft forward. The hotter the combustion and exhaust gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the combustion-gas temperature.
The turbine includes one or more turbine disks (sometimes termed “turbine rotors”), a number of turbine blades mounted to the turbine disks and extending radially outwardly therefrom into the combustion-gas flow path, and rotating turbine seal elements that prevent the hot combustion gases from contacting the turbine shaft and related components. The maximum operating temperature of the combustion gas is limited by the materials used in the turbine. Great efforts have been made to increase the temperature capabilities of the turbine blades, resulting in increasing combustion gas operating temperatures and increased engine efficiency.
As the maximum operating temperature of the combustion gas increases, the turbine disks and turbine seal elements are subjected to higher temperatures in the combustion-gas environment. As a result, oxidation and corrosion of the turbine disks and turbine seal elements have become of greater concern. Alkaline sulfate deposits resulting from the ingested dirt and the sulfur in the combustion gas are a major source of the corrosion, but other elements in the aggressive combustion-and bleed gas environment may also accelerate the corrosion. The oxidation and corrosion damage may lead to premature removal and replacement of the turbine disks and turbine seal elements unless the damage is reduced or repaired.
The turbine disks and turbine seal elements for use at the highest operating temperatures are made of nickel-base superalloys selected for good toughness and fatigue resistance. These superalloys are selected for their mechanical properties. They have some resistance to oxidation and corrosion damage, but that resistance is not sufficient to protect them at the operating temperatures that are now being reached.
The current state of the art is to operate the turbine disks and turbine seal elements without any coatings to protect them against oxidation and corrosion. At the same time, a number of oxidation-resistant and corrosion-resistant coatings have been considered for use on the turbine blades. These available turbine-blade coatings are generally too thick and heavy for use on the turbine disks and turbine seal elements and also may adversely affect the fatigue life of the turbine disks and turbine seal elements. There remains a need for an approach for protecting turbine disks and turbine seal elements against oxidation and corrosion as the operating-temperature requirements of the turbine disks and turbine seal elements increase. This need extends to other components of the gas turbine engine that operate in similar temperature ranges, as well. The present invention fulfills this need, and further provides related advantages.